In a typical cellular communications system, a certain geographical region is divided into regions called cells. The mobile stations (MSs) in each cell are served by a single base station (BS). A BS transmits information to a particular MS (or a group of MSs) in its cell on the radio path referred to as the downlink (DL), while the MSs transmit information to the BS on the radio path referred to as the uplink (UL). The transmissions on the UL and the DL may be on the same time intervals but on different frequency bands (referred to as frequency division duplexing, or FDD), or on the same frequency band but during non-overlapping time intervals (referred to as time division duplexing, or TDD).
In one type of cellular communication system, the transmissions on the DL and UL are based on OFDM (Orthogonal Frequency Division Multiplexing) modulation. In OFDM modulation, the available bandwidth for the radio link (DL or UL) is divided into a large number of smaller-bandwidth units referred to as sub-carriers (SCs), onto which the information to be transmitted is embedded.